The present invention relates to an apparatus employed in a system for treating semiconductor devices, such as a manufacturing system or an examining or inspecting system, to detect a positional shift of a semiconductor wafer, i.e., the central position and the like of the wafer relative to a reference center for positioning the wafer.
In a system for treating semiconductor devices, such as a manufacturing system or an examining or inspection system, a wafer is subjected to a positioning operation with reference to an orientation flat or notch, before the wafer is processed or examined.
In an apparatus for performing such a positioning operation, a wafer is held on a worktable, which is rotated by a motor. Adjacent to the worktable, the light emitting and light receiving members of a photoelectric sensor are arranged to interpose the edge region of the wafer therebetween. Note that "the edge region of a wafer" means a region including the edge and part thereabouts of the wafer.
During rotation of the wafer, the light emitting member of the photoelectric sensor emits light rays, such that the optical axis crosses the wafer edge region. The light receiving member receives part of the light rays which have not been shaded by the wafer, and then outputs electric signals representing the received light rays.
A motor control section receives the output signals of the photoelectric sensor, and the orientation flat or notch of the wafer is detected on the basis of the output signal. Then, a motor is rotated, so that the orientation flat or notch of the wafer is moved to a predetermined position.
Further, the edge region of the wafer is pushed by a positioning pusher to move the wafer in a direction perpendicular to the rotational axis of the motor, so that the center of the wafer is aligned with the rotational axis of the motor. However, when the wafer edge region is pushed by the pusher to align the center of the wafer with the rotational axis of the motor, the wafer may be damaged or stained by an impact applied by the pusher or friction between the wafer and the worktable.
In order to counter such a problem, Jpn. Pat. Appln. KOKAI Publication No. 5-226459 discloses a technique of positioning a wafer without any contact therewith.
In this technique, there are arranged an X-directional-position detecting unit, a Y-directional-position detecting unit, and a mechanism for correcting these units, to observe the wafer edge region. These units and mechanism make the whole structure complicated and expensive. Besides, this technique can be applied to a wafer having an orientation flat, but not to a wafer having a notch.
As described above, positioning a wafer by pushing its edge region damages or stains the wafer. On the other hand, positioning a wafer without contact makes an apparatus complicated and costly.